1. Field of the Invention
The invention relates to a method for producing a product comprising plastic foam in an automatic molding machine.
2. Background Information
Beads are foam particles of small dimensions. Various plastics can be employed in this context, e.g. polystyrene (PS), polyethylene (PE) or polypropylene (PP). The following area of application is envisaged in European Patent No. EP 0664313 of Sep. 9, 1998, for example: polypropylene as well as ethylene and propylene copolymers, comonomers, e.g. alpha alkenes such as propylene, butene, pentene, hexene, octene; furthermore vinyl esters such as vinyl acetate, acrylic acid esters, methacrylic acid esters, maleic acid esters or fumaric acid esters of alcohols.
The beads can have a spherical or some other shape, e.g. a cylindrical shape. The shapes of the beads are determined in part by the way in which they are produced. Irrespective of their shape, the bead diameter is generally used to define the size of the beads. Diameters of 0.5 to 6 millimeters are customary. This does not exclude smaller or larger beads.
A fundamental distinction is drawn, in producing beads, between production in an autoclave and production by extrusion. After production, the beads are held in silos or in some other suitable way before being used.
The beads are joined together in a mold, referred to as an automatic molding machine. This is accomplished by heating the beads at the surfaces until there is a greater or lesser degree of softening, and by compression. Given sufficient melting of the surface and pressure, the beads weld together. Adequate bonding/sintering may occur with less melting and a lower pressure.
Heating not only causes melting of the surface but also expansion. The expansion depends on the type and amount of gas trapped in the beads. The expansion contributes to the build-up of pressure.
Otherwise, the pressure is normally built up by virtue of the fact that the beads are fed into the mold cavity of the automatic molding machine under pressure and that the cavity is completely filled. Even if the mold is ventilated after filling, the beads introduced remain under pressure because the beads compressed by the filling pressure cannot expand in the mold cavity.
A gaseous medium, in particular conveying air, is suitable for feeding in the beads. Conveying air is available in unlimited supply. One blower is sufficient for producing conveying air. The blower draws in ambient air and forces this air through a pipe into the mold cavity of the automatic molding machine. On the way to the mold cavity, the conveying air draws the desired quantity of beads out of a silo, preferably from the bottom.
In the mold cavity, the beads are separated from the conveying air. While the conveying air is supposed to flow into the mold cavity and then back out of the mold, the beads are supposed to remain in the mold cavity. The beads are retained in the automatic molding machine because the exhaust-air opening is of appropriately narrow configuration. That is to say, the diameter of the exhaust-air opening is smaller than the diameter of the beads. In order, at the same time, to allow a relatively large conveying air rate, it is customary to provide a plurality of exhaust-air openings. In practice, it is customary to distribute a multiplicity of openings over the inflow area of the automatic molding machine. EP 0664313 describes an automatic molding machine of this kind. Here, a perforated mold is referred to. That is to say there are a sufficient number of openings to allow the use of the term xe2x80x9cperforated surfacexe2x80x9d.
Once sufficient filling has been achieved, the feed opening of the mold is closed. Superheated steam is then blown through corresponding nozzles into the mold cavity. The superheated steam flows between the beads to the opposite side of the mold cavity and flows out there, like the conveying air before it. During this process, the superheated steam displaces the air which remains between the beads and heats up the surface of the beads. The steam pressure is chosen so that it will overcome the flow resistance of the beads in the mold cavity. The temperature of the steam is furthermore chosen so that within a steam application time (generally up to five minutes) the beads are heated and melt at the surface to an extent such that the bead pressure leads to welding of the beads at the contact surfaces. The necessary pressure is brought about by the pressure filling of the mold cavity and/or by the pressure of the superheated steam and/or by the heating and associated expansion of the beads. The surface temperature of the beads should exceed the so-called glass transition temperature but should remain below the melting temperature of the plastic.
During the process, the openings in the mold are generally replicated on the surface of the moldings produced.
EP 0664313 proposes a concept by means of which replication of the openings is avoided. In this case, a woven wire fabric or a perforated plate is placed against at least one mold wall on the inside, its thickness being 0.2 to 5, preferably 0.5 to 2, millimeters. Metal, such as aluminium or stainless steel, or temperature-stable plastic is to be used as the material for the wire or plate. The woven wire fabric or perforated plate are to be laid loosely in the mold, pressed in, fastened by means of clips or screws or spot-brazed or-spot-welded in. As is customary, the walls of the mold can be provided uniformly with perforations, allowing the superheated steam to spray the interior of the mold uniformly. Since a certain clearance is produced between the surface of the molding and the inner wall of the mold by the inserts, the nozzles cannot be copied on the surface of the molding. On the other hand, the particles resting on the surface are pressed partially into the holes in the fabric or plate, with the result that their structure is impressed upon the surface of the molding. The result is that, instead of receiving irregular impressions, it has essentially a completely regular structure imparted to it.
Structured surfaces are not always advantageous. In the food industry, for example, smooth, easily cleaned surfaces are required for reasons of hygiene. The moldings and automatic molding machines known from EP 0664313 cannot achieve this.
The object on which the invention is based is to improve the surface finish. In this context, the invention sees a conceptual starting-point in the woven fabric lining of the mold or automatic molding machine of the type disclosed by EP 0664313 and also known from earlier publications, even though an unwanted surface structure is produced there.
The invention teaches that this object can be accomplished by a method for producing a product comprising plastic foam in an automatic molding machine with a mold cavity in conformity with the configuration of the product, said method comprising the steps of: providing a woven fabric on at least one surface of said mold cavity, said woven fabric comprising a fabric having a plurality of plies, with a fine-mesh woven fabric ply that faces the interior of said mold cavity upon assembly and a coarse-mesh woven fabric ply that is disposed adjacent said at least one surface of said mold cavity and beneath said fine-mesh woven ply upon assembly to support said fine-mesh woven ply; introducing beads comprising plastic foam into said mold cavity; contacting said beads with a gaseous heat transfer medium, such as superheated steam; said step of contacting comprising: introducing said gaseous heat transfer medium into the mold cavity; flowing said gaseous heat transfer medium along a first surface of said mold cavity; and removing said gaseous heat transfer medium along a second surface of said mold cavity; and removing the product from the mold.
According to the invention, in accordance with one aspect, the desired smooth surface structure is achieved with the aid of a lining comprising a plurality of interconnected woven fabric plies, the foam side of which has a small mesh width, which is selected as a function of the beads being processed and is no longer reproduced as a structure on the surface of the molding. There can also be a plurality of woven fabric plies with a fine mesh width on the foam side.
The use of a woven fabric is admittedly known per se from International Patent Application WO 95/08433 with International Application No. PCT/DE94/01096 filed on Sep. 21, 1994. However, a single-ply woven fabric is not well suited to bringing about the required distribution of the steam. For this reason, the fabric in line with this proposal is given an inherently functional shape as an additional structural feature.
The woven fabric structure used according to the invention on the foam side (referred to below simply as a woven fabric ply on the foam side) is supported by one or more coarser woven fabric plies.
The woven fabric used according to the invention can be composed of metal wires and/or non-metallic threads and the like.
Pleasingly smooth surfaces can surprisingly already be obtained when the mesh width of the woven fabric ply on the foam side is less than half the diameter of the beads employed.
In the application to moldings for the food sector or the like applications with a similar or even higher standard of hygiene, smaller mesh widths are envisaged for the woven fabric ply on the foam side, e.g. 0.001 to 0.5 millimeters, preferably 0.005 to 0.2 millimeters, in the case of beads with a diameter of 3 to 5 millimeters. In the case of other beads, similar (appropriate) limits can be determined by a small number of tests.
Operating automatic molding machines with woven fabrics of this kind used according to the invention also provides another surprise: the boundaries of the beads which were otherwise visible on the surface of moldings produced from foam particles disappear. The surface is even better for various applications, e.g. is easier to clean and also better for application of laminating foils.
The visible boundaries between the beads indicate that the beads have not welded together completely. In severe cases, the beads can be detached easily from the surface of the molding. This is a troublesome phenomenon which is familiar from conventional polystyrene moldings for packaging. The invention attributes this to the fact that some of the water condenses out of the superheated steam onto the metal walls of the mold cavities and remains between the beads in conventional automatic molding machines. This is to be attributed to the fact that the temperature at the walls of the automatic molding machines in the case of conventional molds is significantly below the temperature of the steam, e.g. 80 degrees Celsius during steam treatment. In contrast, the steam is at a temperature of about 120 degrees Celsius and about 1.2 bar, a bar being a unit of pressure or stress, is equal to 105 newton per square meters, even when processing polystyrene. In the case of PP, the difference is even more critical. In this process, steam at about 170 degrees Celsius and 4 to 6 bar is required. As the difference in temperature increases, higher levels of condensate formation must also be expected. In the mold, the water acts as a parting agent between the beads.
It is only the multi-ply woven fabric layers used according to the invention which prevent the occurrence of condensate. The invention attributes this to the following factors: firstly, the woven fabric used according to the invention is extremely good for carrying away superheated steam and condensate compared with other known automatic molding machines. Secondly, the woven fabric used according to the invention has a very low heat absorption compared with conventional devices, with the result that the woven fabric used according to the invention heats up quickly. This applies particularly to the decisive area of contact with the foam. Once the contact surfaces are heated up, there is no longer any risk of condensate formation.
Insofar as condensate formation occurs on the woven fabric ply on the foam side when superheated steam is applied, the condensate is carried along and discharged by the superheated steam flowing in behind.
Condensate which forms along the path of the superheated steam from the woven fabric ply on the foam side to the outlet from the automatic molding machine is optionally extracted by the application of a vacuum on completion of treatment with superheated steam.
It is generally sufficient to set up a steam chamber in the exhaust-steam section (path for the exiting superheated steam) and to apply the vacuum there.
The permeability according to the invention of the woven fabric used can be determined from the flow resistance. The studies depicted in FIGS. 5 and 6 were carried out
a) in the case of FIG. 5, at an outside temperature of 22 degrees Celsius, an atmospheric pressure of 988 mbar [1 millibar being equal to 100 newtons per square meter] and with a test piece diameter of 100 millimeters, and
b) in the case of FIG. 6, at an outside temperature of 23 degrees Celsius, an atmospheric pressure of 1018 mbar and with a test piece diameter of 100 millimeters.
The woven fabric used according to the invention and shown in FIG. 5 has a total of 4 plies with a woven fabric ply on the foam side with a mesh width of 0.005 millimeters.
The woven fabric used according to the invention and shown in FIG. 6 likewise has 4 plies with a woven fabric ply on the foam side with a mesh width of 0.14 millimeters.
The woven fabric used according to the invention allows significantly more intense cooling than in the case of conventional automatic molding machines.
Fundamental advantages in terms of cooling are obtained by virtue of the fact that the mass of the woven fabric used according to the invention is significantly lower than that of comparable systems.
Moreover, cooling of the woven fabric used according to the invention and of outer layers of the molding formed can be achieved very quickly by blowing in cooling air, for example. The outer layers of the molding formed determine its dimensions.
The woven fabrics used according to the invention are also suitable for the application of some other coolant. The coolant can also be water. Particularly good conditions can be obtained by spraying the resulting molding with cooling water.
During initial cooling, the moldings must remain in the automatic molding machine. The cooling time has a significant influence on the cycle time of manufacture. Cooling in accordance with the invention allows the cycle time to be reduced accordingly.
The production cycle ends with ejection of the moldings formed from the automatic molding machine. The surfaces which determine the shape should then be at a temperature below the glass transition point. In the case of polystyrene, the ejection temperature is 40 to 90 degrees Celsius and, in the case of PP; it is 70 to 100 degrees Celsius.
The smooth woven fabric surface according to the invention is advantageously also favourable for mold release.
The thread and wire diameter is generally obtained from the mesh width. It is also possible to make a free selection of the thread and wire thickness/diameter as a function of the mesh width when designing the woven fabric plies used according to the invention. It is advantageous if the thread and wire thickness/diameter of the woven fabric ply on the foam side is equal to or less than 10 times the mesh width. Even greater advantages as regards the surface and as regards permeability to air and steam are obtained if the thread and wire thickness/diameter is equal to or less than five times the mesh width. It is also possible to consider threads and wires with a thickness/diameter which is equal to or less than the mesh width.
According to the invention, the woven fabric ply on the foam side is supported by a coarser woven supporting structure which overall has a larger mesh width, preferably 1.5 to 20 times the mesh width, and a larger thread and wire thickness, preferably also 1.5 to 20 times the thread and wire thickness in the woven fabric ply/woven fabric structure on the foam side. Good flow conditions are obtained in the woven fabric used according to the invention especially if:
a) the mesh width of the woven supporting structure is greater than the mesh width of the ply on the foam side plus 2.5 times the thread or wire thickness of the ply on the foam side, and/or
b) the woven fabric plies used according to the invention are inserted in such a way as regards their direction of weaving that the directions of weaving of two adjacent woven fabric plies differ by 90 degrees.
Weaving is defined by the warp and weft threads or warp and weft wires. This results in strip stock. The longitudinal direction of the strip is referred to below as the weaving direction.
The weaving direction remains the same even if lengths are cut or sections cut out of a strip of fabric for the woven fabric used according to the invention. The weaving direction can be identified from the warp and weft threads or warp and weft wires. The distinction is easy to make because the warp and weft can be distinguished in the woven fabric. To achieve the offset of 90 degrees in accordance with the invention, care must merely be taken to ensure that the threads or wires which have been worked in the same way in two adjacent woven fabric plies used according to the invention cross one another. A 90-degree offset results in exactly perpendicular crossing.
However, crossing at an angle can also be performed.
Where there are more than two woven fabric plies used according to the invention, the above conditions preferably apply in relation to the respective woven fabric plies used according to the invention resting upon one another. The respective smaller dimensions relate to the woven fabric ply used according to the invention that is closer to the foam. Individual fine-mesh plies in the woven fabric structure on the foam side and individual coarse-mesh plies in the woven supporting structure can optionally be repeated.
Three or more woven fabric plies are advantageous for the woven fabric used according to the invention. Superheated steam can be distributed in an excellent manner in the woven fabric used according to the invention through the coarser woven fabric plies used according to the invention.
The metallic woven fabric plies used according to the invention can be welded to one another. Induction welding is advantageous in this context. Induction welding can be monitored very well, and there is therefore essentially no risk that individual wires will be burnt through. Metallic woven fabric plies used according to the invention can also be sintered together. Sintering is similar to welding. The contact surfaces are heated and pressed together. While heating is significantly more intense in the case of welding than sintering, the pressure involved in welding is significantly lower than in the case of sintering. Brazing of the metallic woven fabric plies used according to the invention is furthermore worth considering. All the woven fabric plies used according to the invention can be bonded or held in the desired shape and position with fastening or clamping elements. Various means, e.g. screws or rivets, are suitable as fastening or clamping elements.
Non-metallic woven fabric plies used according to the invention can be composed of carbon fibers, for example. Carbon fibers have a high strength and low thermal conductivity. It can be advantageous to combine metallic woven fabric plies used according to the invention with non-metallic woven fabric plies used according to the invention. Metallic woven fabric plies used according to the invention especially are suitable for fastening by means of pressing or crimping.
The woven fabrics used according to the invention can be prepared as a composite material and inserted into the mold. It is also possible to delay connecting the woven fabric plies used according to the invention until they are in the mold. Although connection of the woven fabric plies used according to the invention over a partial area and/or connection of the woven fabric used according to the invention over a partial area with the walls of the automatic molding machine which lie behind it is possible, connection over the full area can provide advantages.
Surprisingly, the formation of condensate from the superheated steam on the outflow side when woven fabrics used according to the invention are used in accordance with the invention is very much less than in conventional automatic molding machines. The formation of condensate likewise impairs the quality of the products. The invention also improves cycle times. These can be shortened. In the case of moldings which are 100 millimeters thick, cycle times of 2 to 5 minutes are easily possible.
Woven wire fabrics made of steel, especially with alloying components such as chromium, nickel and molybdenum which result in high-grade steel, are favorable.
The total thickness of the woven fabric layer used according to the invention can be affected by the required air/steam flow and/or by stability considerations. The thicker the threads/wires and the larger the mesh widths, the more easily the superheated steam can flow through. The thickness may be up to 15 millimeters, especially 3 to 5 millimeters.
The woven fabrics used according to the invention optionally rest on a continuous wall or a wall provided with through openings, and/or the woven fabrics used according to the invention are designed to be completely or partially self-supporting and/or rest completely or partially on supporting surfaces of the automatic molding machine. The associated walls of the automatic molding machine are preferably made from metal, e.g. aluminum or sintered aluminum, or from ceramics. The walls can also be provided with a porous, gas- and water-permeable coating which is conducive to steam application and steam removal.
The woven fabric used according to the invention is optionally in the form of a panel, flat or shaped. The deformation of the woven fabric plies used according to the invention is also important and applicable to automatic molding machines, irrespective of the number according to the invention of woven fabric plies used.
The deformation of metallic woven fabrics used according to the invention can be achieved by means of a suitable press by pressing and/or deep drawing. The press has a die and a punch which are shaped to match the desired shaped surface.
Deformation can be accomplished cold but is significantly easier if the metallic woven fabrics used according to the invention are heated.
The woven fabric layers are deformed individually or jointly.
In the case of joint deformation, it is advantageous to secure on the foam-side ply a bending aid of approximately the same thickness as the woven fabric used according to the invention, ensuring that the foam-side ply forms approximately the neutral axis in the deformed composite material composed of the woven fabric and the bending aid. In this case, use is made of the knowledge that the neutral axis undergoes the least deformation in the course of bending deformation. Approximately the same thickness includes deviations of plus/minus 50% in the thickness of the bending aid from the thickness of the woven fabric. The auxiliary bending material is preferably a non-foamed film/panel made of PE (polyethylene) and/or PP (polypropylene).
The PE/PP material simultaneously forms a protective film. It is advantageous to arrange protective films on all the outer surfaces of the woven fabric which are subjected to load during deformation.
Using a relatively high tool hardness in the areas of contact with the woven fabric used according to the invention also helps to reduce stress on the outer surfaces of the woven fabric. This prevents the woven fabric used according to the invention from catching in the contact surface of the tool.
Before deforming the woven fabric used according to the invention and/or after connecting the woven fabric plies used according to the invention, heat treatment of the woven fabric plies or of the composite material can reduce stresses in the material and facilitate subsequent deformation.
Experience has shown that the automatic molding machines require cleaning after 500 to 1000 cycles. The feed and discharge lines are then so soiled that cleaning is required. The woven fabric used according to the invention opens up the possibility of extending the running time (operating intervals) until cleaning is next required or significantly increasing the number of cycles. The woven fabrics used according to the invention are favourable for reversing the direction of steam application. Changing the direction of steam application expels the dirt particles which have settled in a woven fabric. Expulsion can take place at the end of the customary running time. However, expulsion can also take place after a short running time and, in an extreme case, after every second cycle. The technique according to the invention for extending the running time of automatic molding machines can advantageously also be applied irrespective of whether the fabric has multiple plies and can also be applied to other automatic molding machines.
Because of the possibility of soiling of the fabric, a woven filter fabric is optionally provided on the inlet side. The dirt particles collect in the filter fabric.
Another advantage of the woven fabrics used according to the invention comes into play when cross-flow steam application is employed. According to the invention, a molding charge is steam-treated from several directions. This can take place simultaneously or at alternate intervals. Cross-flow steam application makes the quality of welding of the beads more uniform. When applying steam in just one direction, the temperature of the steam falls during passage through the material owing to heat transfer. Consequently, there is also a gradient as regards the surface temperature of the beads. Cross-flow steam application ensures that those areas which were previously at the end of the steam path are reached by fresh steam. Cross-flow steam application is achieved by providing additional walls of the automatic molding machine with steam application openings and, if appropriate, exhaust-steam openings with corresponding inlet and outflow lines. To control the individual lines appropriate slide valves are provided in the inlet lines and/or outlet lines.
Cross-flow steam application is also advantageous if uniform steam distribution has not taken place in the mold cavity. Those areas which have been inadequately penetrated by steam are then reached by the steam through cross-flow steam application.
Cross-flow steam application is also important for automatic molding machines independently of the woven fabric according to the invention.
The above-discussed embodiments of the present invention will be described further herein below. When the word xe2x80x9cinventionxe2x80x9d is used in this specification, the word xe2x80x9cinventionxe2x80x9d includes xe2x80x9cinventionsxe2x80x9d, that is the plural of xe2x80x9cinventionxe2x80x9d. By stating xe2x80x9cinventionxe2x80x9d, the Applicants do not in any way admit that the present application does not include more than one patentably and non-obviously distinct invention, and maintains that this application may include more than one patentably and non-obviously distinct invention. The Applicants hereby assert that the disclosure of this application may include more than one invention, and, in the event that there is more than one invention, that these inventions may be patentable and non-obvious one with respect to the other.